1. Technical Field of the Invention
The present invention relates to semiconducting ceramic compounds which have negative resistance-temperature characteristics with critical temperatures.
2. Description of the Related Art
Negative temperature coefficient semiconducting ceramics (hereinafter referred to as NTC ceramics) whose electrical resistance is high at room temperature and drops with temperature increase are known in the prior art. Examples of materials for producing such NTC ceramics are oxides of transition elements which have a spinel crystalline structure and mainly contain a few elements selected from aluminum, manganese, iron, nickel, cobalt and copper, as well as cobalt-containing rare-earth oxides which have a perovskite crystalline structure and mainly contain LaCoO.sub.3.
NTC ceramic devices produced by forming electrodes on each piece of an NTC ceramic material consisting essentially of the oxides of some of the aforementioned transition elements are used as rush current preventing devices for switching regulators or for the protection of motors or halogen lamps, or as temperature-sensitive devices for temperature sensing or for detecting the surface of a liquid, for instance. On the other hand, NTC ceramic devices produced by forming electrodes on each piece of an NTC ceramic material consisting essentially of a cobalt-containing rare-earth oxide are commonly used as rush current preventing devices for switching regulators or for the protection of motors or halogen lamps, for instance.
Also known in the prior art are critical temperature resistor semiconducting ceramics (hereinafter referred to as CTR ceramics) whose electrical resistance sharply drops beyond specific temperatures. Examples of materials for producing such CTR ceramics are vanadium dioxide (VO.sub.2) and nickel-containing rare-earth oxides, the latter including SmNiO.sub.3, NdNiO.sub.3, PrNiO.sub.3 and EuNiO.sub.3, for instance.
CTR ceramic devices produced by forming electrodes on each piece of a CTR ceramic material consisting essentially of VO.sub.2 are used as temperature-sensing devices for fire-alarm systems.
Although the aforementioned NTC ceramics composed essentially of the oxides of some of the transition elements and the NTC ceramics consisting essentially of the cobalt-containing rare-earth oxides have negative resistance-temperature characteristics, neither of them exhibit critical temperatures at which their resistance sharply drops. It has therefore been necessary to use the conventional NTC ceramic devices together with a controlling microcomputer circuit when applying them to on-off switching operations, and this has resulted in large component sizes and high costs of materials.
While the CTR ceramics consisting essentially of VO.sub.2 have the characteristic that their resistivity drops from 10.sup.4 ohm-centimeters to 10 ohm-centimeters in a temperature range of 60.degree. C. to 80.degree. C., they are not in a stable phase at room temperature. This develops a problem in that these CTR ceramics can be destroyed when brought into contact the air or moisture. Furthermore, since their critical temperatures are restricted within the range of 60.degree. C. to 80.degree. C., their applications are limited to temperature-sensing devices for fire-alarm systems.
The resistance of the CTR ceramics consisting essentially of the aforementioned nickel-containing rare-earth oxides sharply drops beyond specific temperatures (metal-to-semiconductor phase transition temperatures) as discussed in a paper presented by J. B. Torrance, et al. (FIGS. 1 and 2 on page 8,210 of Physical Review B45 14!, 1990) Such characteristics of the CTR ceramics, particularly SmNiO.sub.3, NdNiO.sub.3 and PrNiO.sub.3, are also discussed in a paper presented by P. Lacorre, et al. (FIG. 4 on page 225 of Solid State Chemistry, 1991)
Although these papers give resistance values of different samples in ohms at about their phase transition temperatures, neither of them indicates specific shapes of the samples. It is therefore impossible to know their resistivities or conductivities, especially at room temperature (25.degree. C.), from these papers. The inventors of the present invention analyzed the crystalline structures of the same samples using the X-ray diffraction method and succeeded in identifying NdNiO.sub.3 and PrNiO.sub.3 in the samples. An analysis of the sample containing SmNiO.sub.3 revealed a diffraction pattern seemingly produced by SmNiO.sub.3, but there was found a diffraction pattern of NiO as well. Therefore, it was impossible to determine that the CTR characteristics of that sample were caused by SmNiO.sub.3 alone.
Substituting lanthanum (La) for 30% of neodymium (Nd) contained in NdNiO.sub.3 reduces its phase transition temperature from -70.degree. C. to -170.degree. C. as discussed in a paper presented by J. B. Garcia-Munoz, et al. (FIG. 1 on page 15,198 of Physical Review B5 21!, 1995) However, it is not certain whether the phase transition temperature can be altered by substituting a rare-earth element rather than lanthanum (La) for the neodymium (Nd).